1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more particularly to sensing images using adjustable modulation transfer functions (MTFs).
2. Description of the Related Art
Imaging systems are commonly employed in many different fields, including machine vision, microscopy, photography, and semiconductor inspection. Imaging systems are typically composed of an optical system, an imaging device, and some form of computer processing and/or analysis. In this context, imaging can be considered a process or series of operations performed on an object or specimen in order to produce the final image, and may include, for example, receipt of the image by an imaging device, removing errant portions of the image by filtering or processing, and so forth. Each segment of the imaging system can perform one or more operations on the object or specimen.
A block diagram of certain components making up a typical imaging process is presented in FIG. 1. Circular elements in FIG. 1 represent the object or specimen 101 or the detected image 105 of the object, while rectangles represent operations on the object 101. In an imaging process, optical system 102 produces a continuous image of an object or specimen 101. A solid state imager 103, formed of a charge diffuser 106 and image array 107, may change the continuous image or optical image into either an electronic image or an array of values convertible to an electronic image. This electronic image may then be processed at point 104 before rendering the final detected image 105.
Problems associated with the system of FIG. 1, particularly in a high speed scanning or inspection of the object or specimen 101, can include aliasing. Aliasing is the effect occurring when analog information is sampled into pixels, and the resultant pixelized image differs, sometimes significantly, from the original analog signal. Prior approaches to prevent aliasing use optical techniques to modify the resolution of the optical image on the sensor, or use image processing algorithms. These approaches either have limited effectiveness, or adversely affect performance of the imaging system.
One tool that has been employed to describe the performance of an imaging process or system such as that shown in FIG. 1 is the Modulation Transfer Function (MTF). The MTF mathematically describes the contrast at which different spatial frequencies of the object may be transferred. High spatial frequencies correspond to fine detail in the object or specimen 101. An MTF of 100% at a particular spatial frequency transfers that frequency from the object or specimen with full contrast. The resolution of an imaging process or system may be readily expressed in terms of one MTF or a combination of MTFs, because the MTF of various imaging operations can be multiplied together to produce the MTF of the detected image 105. The MTF for an imaging process can be represented by the following equation:MTFImage=MTFOptical*MTFDiffusion*MTFArray*MTFProcessing  (1)MTFOptical represents the MTF of the optics of the optical system, while MTFDiffusion is the MTF of the charge diffuser 106. MTFArray is the MTF created by the discrete nature of the pixel sampling by the sensor or sensing array, while MTFProcessing is the MTF of the image processing subsequent to image formation on the sensing array. Multiplication of the MTFs in the manner shown in Equation (1) can have two adverse effects on detected image 105. First, the contrast of each spatial frequency component degrades with each operation on the object 101. In many cases it is desirable to minimize this degradation and produce a detected image 105 that resembles object or specimen 101 as closely as possible with respect to contrast. Reducing the contrast of specific spatial frequency components in the original object 101 that are outside the region of interest may be advantageous. However, such contrast reduction may cause other spatial frequency components to be more pronounced in the detected image 105.
Second, aliasing remains an issue when using sampling detectors. For example, if the sampling frequency of the solid state imager or sensor 103 is less than the Nyquist frequency of the object or specimen 101, aliasing can occur. Aliasing tends to degrade the detected image 105 and the presence of aliasing causes difficulties in using the detected image 105 in subsequent processing.
Aliasing can be addressed by modifying the MTFs of the optical system 102, the charge diffuser 106, or the image array 107 operations. Several common techniques may be employed to maintain the contrast of the spatial frequencies in the object or specimen 101, or alternately to reduce the contrast of specific spatial frequencies present in the object or specimen 101 by altering the optical system MTFOptical. One style reduces the optical resolution using a spatial filter or apertures to reduce the imaging numerical aperture. This approach limits the magnifications and numerical apertures that can be used, and discards light energy associated with the high resolution information. Discarding light energy in a high speed inspection can reduce throughput, or in other words require a longer time period to scan a given region. Another optical anti-aliasing style uses optical filters, such as birefringent plates, to limit optical resolution without discarding light energy. This technique reduces the contrast of the highest spatial frequencies. Such filtering tends to modify the light itself before a solid state sensor converts the light into an image array. Devices operating according to this anti-aliasing style require additional hardware, are generally not effective over a range of wavelengths, may have direction asymmetry, and are designed to work at fixed resolutions. Such filtering can also require operation at wavelengths above 400 nm, while many newer systems operate at wavelengths below 400 nm. Once the solid state imager converts the continuous optical image to a discrete image array, spatial frequency information transferred from the object or specimen is permanently altered.
The image may be modified subsequent to this alteration using image processing algorithms 104. These algorithms attempt to reduce the impact of aliasing by altering the image processing 104 MTFprocessing. However, such algorithms may not effectively address aliasing and other image artifacts produced within the solid state imager when forming the image array. General aliasing artifacts are impossible to remove after the image is transferred to the solid state sensor of the optical imaging system, as the artifacts are generally indistinguishable from true image detail. Due to the strong resemblance between artifacts and image detail, no effective software anti-aliasing filters are available.
The solid state imager itself may be used to control the MTF of the final image. U.S. Pat. No. 6,265,736 presents a design that uses the image array operation of the solid state sensor to modify the MTF. The '736 patent uses a pixel “binning” approach which spatially combines pixels into a larger pixel by combining them in “bins.” This approach modifies the MTFArray portion of the imaging process. The '736 type design offers limited control and adjustment of the MTF because an integer number of pixels must be combined. For systems that operate at high speed or at sensor pixel sizes larger than about ⅛th the optical resolution, this approach tends to produce unacceptable results.
Thus while it can be advantageous to employ a tunable MTF in imaging a specimen or object, certain design limitations exist when using the MTF that can distort or compromise the image ultimately rendered. It would be desirable to have the ability to control the sensor MTF to eliminate or dramatically reduce the effects of aliasing and the adverse effects associated with altered image contrast, without adversely impacting system performance, cost, or complexity.